Individually passing branches can fail when combined. A merge queue tests the prospective integration state before accepting changes.
Before you start
You should understand commits, branches, the working tree and the staging area. Draw the commit graph before changing history. Try commands in a disposable repository with a clean working tree so you can observe exactly which references and files each operation changes.
The practical goal is to reason through this situation: Two passing changes depend on incompatible interface assumptions. Read the walkthrough first, then try the interview exercise before opening its answer. The important part is explaining the decision and its consequences, rather than remembering a definition alone.
Step-by-step walkthrough
Step 1: Identify integration races
Two passing branches can rely on incompatible assumptions.
Step 2: Test prospective combined state
Validate the state that would actually enter main.
Step 3: Handle failed combinations
Report the problematic integration rather than trusting earlier branch checks.
Worked scenario
Two passing changes depend on incompatible interface assumptions.
Branch A renames a helper, while B adds a new use of its old name. Both pass against their earlier bases, but the combined tree fails. A merge queue checks their prospective integration with current main, reducing this race; required checks still need meaningful coverage.
Common mistake
Checking only each branch’s earlier base leaves a race with main’s movement.
Verify the behavior
Construct the incompatible pair and verify combined validation rejects it.
Interview exercise
Protect a busy main branch.
Answer and reasoning
Validate the combined state with required checks and ensure failures identify which queued combination introduced the conflict.
Continue learning
Compare the scenario with the Git and CI/CD interview questions and test your understanding with the Git and CI/CD MCQs. For terminology and implementation details, consult the reference material.